I use silk for sky these days. I used to use various painted cotton fabrics, then sateen which is nicely shiny, but silk has a luminosity that can't be matched. I have several different weights, from sheer organza to nicely slubby noil and even some very heavy, drapey satin weave.
I have to dye the colors I'm looking for, which isn't hard. I have a handful of Colorhue dyes which are very easy to use. They work without chemicals, mix easily, dilute easily and produce brilliant colors. This piece though, I used Inkodye which comes out of the jar a gluey, muddy color that develops in sunlight.
If I wanted some control over results, the Inkodye would drive me crazy. But I like the surprise of the process, and the colors are always interesting.
This is the beginning for a pair of pieces.